This invention relates generally to color imaging and more particularly to the use of plural exposure and development steps for such purposes.
One method of printing in different colors is to uniformly charge a charge retentive surface and then optically expose the surface to information to be reproduced in one color. This information is rendered visible using marking particles followed by the recharging of the charge retentive surface prior to a second exposure and development. This recharge/expose/and develop process may be repeated to subsequently develop images of different colors in superimposed registration on the surface before the full color image is subsequently transferred to a support substrate. The different colors may be developed on the photoreceptor in an image on image development process, or a highlight color image development process (image next-to image). The images may be formed by using a single exposure device, e.g. ROS, where each subsequent color image is formed in a subsequent pass of the photoreceptor (multiple pass). Alternatively, each different color image may be formed by multiple exposure devices corresponding to each different color image, during a single revolution of the photoreceptor (single pass).
Several issues arise that are unique to the image on image process of creating multi-color images in the attempt to provide optimum conditions for the development of subsequent color images onto previously developed color images. For example, during a recharge step, it is important to level the voltages among previously toned and untoned areas of the photoreceptor so that subsequent exposure and development steps are effected across a uniformly charged surface. The greater the difference in voltage between those image areas of the photoreceptor previously subjected to a development and recharge step; those image areas subjected to a development step, but not yet subjected to a recharge step; and those bare non-developed, untoned areas of the photoreceptor, the larger will be the difference in the development potential between these areas for the subsequent development of image layers thereon.
Another issue that must be addressed with the image on image color image formation process is the residual voltage that exists across the toner layer of a previously developed area of the photoreceptor. Although it may be possible to achieve voltage uniformity by recharging previously developed photoreceptor areas to the same voltage level as neighboring bare areas, this residual toner voltage (V.sub.t) prevents the effective voltage above any previously developed toned areas from being re-exposed and discharged to the same level as neighboring bare photoreceptor areas which have been exposed and discharged to the actual desired voltage levels. The problems become increasingly severe as additional color images are subsequently exposed and developed thereon. Color quality is severely threatened by the presence of the toner charge and the resultant voltage drop across the toner layer. The change in voltage due to the toned image can be responsible for color shifts, toner spreading at image edges, and loss in latitude affecting many of the photoreceptor subsystems. Thus, it is ideal to reduce or eliminate the residual toner voltage of any previously developed toned images.
Prior attempts to address one or more of these issues have introduced a variety of secondary problems, each having an adverse effect on the image on image color image formation process. For example, copending application for U.S. Patent entitled "Method and Apparatus for Reducing Residual Toner Voltage", U.S. patent application Ser. No. 08/347,616, filed by a common assignee as the present application, discloses a voltage sensitive recharge device used for the recharging steps during a color image formation, whose graph of the output current (I) to the charge retentive surface as a function of the voltage to the charge retentive surface (V) has a high (I/V) slope. The high I/V slope recharge device disclosed having an AC voltage supplied thereto, enables an extended time for neutralization to occur at the top of the toner layers. However, this system causes a significant amount of excess current to pass through the toner layer, severely impairing the subsequent attempt to transfer the image from the photoreceptor.
Another recharging method is described in application for Japanese Patent No. Hei 1-340663, Application date Dec. 29, 1989, Publication date Sept. 4, 1991, assigned to Matsushita Denki Sangyo K. K. This reference discloses a color image forming apparatus wherein a first and second charging device are used to recharge a photoconductor carrying a first developed image, before exposure and development of a subsequent image thereon. The potential of the photoconductor is higher after passing the first charging device than after passing the second charging device. This reference teaches that the difference in voltage applied by the first and second charging devices to the toner image and photoreceptor surface is set to a relatively high level, to insure that the polarity of the toner image is reversed after passing and having been charged by both devices. The effect of this teaching is to reduce the residual voltage in the image areas which becomes more severe when applying color toners onto previously developed color toners, and also to prevent toner spray or spread during the exposure process. Toner spray is a phenomena caused when the photoconductor carrying the first toner image is recharged to a relatively high charge level and then exposed for the second image development. The toner of the first image tends to spray along its edges into the subsequently exposed areas which have a relatively lower charge level. By reversing the polarity of the toner as taught in this reference, toner spray is prevented, as the reversed polarity toner is no longer attracted to the exposed areas.
When a substantial amount of toner charge at the top of a previously developed toner layer is reversed in polarity during recharge, however, a different problem of a serious nature develops. Since the prior toner image is now predominantly of an opposite polarity to both the background bare areas and the incoming color toner to be developed thereon, an interaction occurs among these three separate and distinctly charged regions. Particularly, the now-positively charged toner layer is attracted to the negatively charged background areas and the negatively charged toner of the incoming color image. Thus, the positively charged toner of the first image tends to splatter within the developed image and also into neighboring bare background regions. This occurrence has been titled the "under color splatter" defect (UCS) and is the cause for the unwanted blending of colors and the spreading of colors from image edges into background areas. Consequently, color clarity is severely impacted.
Copending application for U.S. Patent entitled "Split Recharge Method and Apparatus for Color Image Formation", U.S. patent application Ser. No. 08/347,617, filed by a common assignee as the present application, discloses a recharge method which attempts to solve the UCS problem. This reference discloses a split recharge configuration wherein a first corona generating device recharges the charge retentive surface having a developed image thereon to a higher absolute potential than a predetermined potential, and then a second corona generating device having an AC voltage supplied thereto recharges the surface to the predetermined potential. The amount of difference in the photoreceptor surface potential after being recharged by the first corona recharge device and the second corona recharge device is defined as the "voltage split". The alternating current generated from the second recharge device substantially neutralizes the electrical charge associated with the image. The teachings of this disclosure also enable a reduced residual toner voltage (V.sub.t) associated with the toned image, as the amount of voltage split applied is directly proportional to the amount of V.sub.t reduction realized. However, in order to avoid the toned image charge from reversing its polarity (to avoid UCS occurrence), the amount of voltage split that can be applied is limited, which in turn limits the amount of residual toner voltage reduction that can be achieved. Furthermore, the process of substantially neutralizing the top toner layer in the attempt to prevent the toner charge from reversing its polarity causes large amounts of current to be driven through the toner layer and into the photoreceptor, making transfer of the image from the photoreceptor and optimization of image quality a difficult process.
Based on the foregoing, a highly reliable and consistent manner of recharging the photoreceptor to a uniform level is needed, whereby the residual voltage on previously toned areas is minimized and the undercolor splatter defect is prevented. Furthermore, a recharging process is needed whereby large amounts of current are not driven through the toner layers, causing difficulty to the subsequent transfer process.
The following references may be found relevant to the present disclosure.
U.S. Pat. No. 4,791,452 relates to a two-color imaging apparatus wherein a first latent image is formed on a uniformly charged imaging surface and developed with toner particles. The charge retentive surface containing a first developed or toned image, and undeveloped or untoned background areas is then recharged by a scorotron charging device prior to optically exposing the surface to form a second latent electrostatic image thereon. An electrical potential sensor detects the surface potential level of the drum to ensure that a prescribed surface potential level is reached. The recharging step is intended to provide a uniformly charged imaging surface prior to effecting a second exposure.
U.S. Pat. No. 4,819,028 discloses an electrophotographic recording apparatus capable of forming a clear multicolor image including a first visible image of a first color and a second visible image of a second color on a photoconductive drum. The electrophotographic recording apparatus is provided with a conventional charger unit and a second corona charger unit for charging the surface of the photoconductive drum after the first visible image is formed thereon so as to increase the surface potential of the photoconductive drum to prevent the first visible image from being mixed with a second color and also from being scratched off from the surface of the photoconductive drum by a second magnetic brush developing unit.
U.S. Pat. No. 4,761,669 relates to creating two-color images. A first image is formed using the conventional xerographic process. Thus, a charge retentive surface is uniformly charged followed by light exposure to form a latent electrostatic image on the surface. The latent image is then developed. A corona generator device is utilized to erase the latent electrostatic image and increase the net charge of the first developed image to tack it to the surface electrostatically. This patent proposes the use of an erase lamp, if necessary, to help neutralize the first electrostatic image. A second electrostatic image is created using an ion projection device. The ion image is developed using a second developer of a different color.
U.S. Pat. No. 4,033,688 discloses a color copying apparatus which utilizes a light-lens scanning device for creating plural color images. This patent discloses multiple charge/expose/develop steps.
U.S. Pat. No. 4,833,503 discloses a multi-color printer wherein a recharging step is employed following the development of a first image. This recharging step, according to the patent is used to enhance uniformity of the photoreceptor potential, i.e. neutralize the potential of the previous image.
U.S. Pat. No. 4,660,059 discloses an ionographic printer. A first ion imaging device forms a first image on the charge retentive surface which is developed using toner particles. The charge pattern forming the developed image is neutralized prior to the formation of a second ion image by a corona generating unit and an erase lamp.
U.S. Pat. No. 5,208,636, discloses a printing system wherein charged area images and discharged area images are created, the former being formed first and the latter being proceeded by a recharging of the imaging surface.
U.S. Pat. No. 5,241,356 discloses a multi-color printer wherein charged area images and discharged area images are created, the former being formed first, followed by an erase step and a recharge step before the latter is formed. An erase lamp is used during the erase step to reduce voltage non-uniformity between toned and untoned areas on a charge retentive surface.
U.S. Pat. No. 5,258,820 discloses a multi-color printer wherein charged area images and discharged area images are created. An erase lamp is used following development of a charged area (CAD), and a pre-recharge corona device is used following development of a discharged area (DAD) and prior to a recharge step, to reduce voltage non-uniformity between toned and untoned images on a charge retentive surface.
Copending application for U.S. patent application Ser. No. 08/346,708, filed by a common assignee as the present application, discloses a corona recharge device for recharging the photoreceptor containing at least one previously developed color image, to a voltage level intermediate to the background areas and the image areas, to keep wrong-charge toner developed in the background areas at a charge level distinct from the toner developed in the image areas so that the wrong-charge background toner does not transfer to a support substrate with the image.
A number of commercial printers employ the charge/expose/develop/recharge imaging process. For example, the Konica 9028, a multi-pass color printer forms a single color image for each pass. Each such pass utilizes a recharge step following development of each color image. The Panasonic FPC1 machine, like the Konica machine is a multi-pass color device. In addition to a recharge step the FPC1 machine employs an AC corona discharge device prior to recharge.